A numerical study on the influence of curvature ratio and vegetation density on a partially vegetated U-bend channel flow. (February 2021)

• Record Type: Journal Article
• Authors: Wang, Mingyang; Avital, Eldad; Korakianitis, Theodosios; Williams, John; Ai, Kaiming
• Is Part Of: Advances in water resources. Volume 148(2021)
• Publication Date: 2021-02
• Subjects: Vegetation patch (VP) flow -- U-bend channel -- Helical flow -- Curvature ratio (CR) -- Solid volume fraction (SVF) -- Drag coefficient; Hydrology -- Periodicals; Hydrodynamics -- Periodicals; Hydraulic engineering -- Periodicals; 551.48
• DOI: 10.1016/j.advwatres.2020.103843
• Languages: English
• ISSNs: 0309-1708

Abstract

Highlights: Curvature ratio of a partially vegetated U-bend channel dominants the main flow and helical flow patterns. A vegetation patch increases the possibility of scouring in the leading region but protect the downstream region. Drag coefficients of vegetation patch vary with curvature ratio. The dominant frequency of vegetation drag signals decrease with the rising of vegetation density, a dimensionless number is proposed to link these two variables. Abstract: Aquatic vegetation dramatically shifts the main flow, secondary flow and turbulent structures in a meandering channel. In this study, hydrodynamics in a bending channel with a vegetation patch (VP) has been numerically studied under the variation of curvature ratios ( CRs =0.5, 1.0, 1.5, 2.0) and the vegetation density i.e. Solid Volume Fractions (SVF=1.13%, 4.86%). Both effects on vegetation shear flow, helical flow, bed shear stress and bulk drag coefficients are studied in twelve cases by using Ansys Fluent package. Unsteady Reynolds Averaging Navier-Stokes (URANS) framework coupled with the Reynolds Stress turbulence Model (RSM) and Volume Of Fluid (VOF) approach is successfully applied to predict the entire flow field including multi-circulation cells as well as the free surface. The conclusions are summarized as three points. Firstly, an increase of CR moves the main circulation cell and thalweg's location towards the outer bank, while decreasing the drag coefficients in streamwise and spanwise. However, the CR weakly affects the normalised shear flow velocity profiles and dominant eddy frequencies downstream of the VP. Secondly, the trend of the dominant shedding frequency to fall with the increase of SVF that has been known only for SVF<3.4% is extended up to 10.4%. Furthermore, an opposite trend is found between the frequency and SVF for 10.4%<SVF<20%. Thirdly, a newly proposed patch dimensionless frequency number, S t p SVF N, links Stp and SVF, where N is the number of stems in the patch. This number stays almost constant for each case series regardless of the variation of SVF (for SVF<10.4%). We also conclude that S t p SVF N is strongly determined by the patch shape factor, mildly influenced by the patch Reynolds number, but it excludes the influence of the SVF and N . The insights from the present study unveil the complicated eco-hydro-morphic interactions among the bio-mass density, turbulent flow and channel meanders' variation. It provides a better understanding of natural bending river systems' development and fundamentals for the recovery of urban channel ecosystems by vegetated re-meandering.